Lubricating Device

I am the original owner of a 1967 Pontiac GTO with a 400 ci engine and a 2bbl carburetor. This car was built long before onboard computers and anti-pollution devices. My car seems to be running very hot but unfortunately I do not have a water temperature gauge. I have lubricated the manifold heat riser to make sure it is working properly so hot exhaust gases will be sent directly out the tailpipe after the car warms up. My question is what would be wrong with replacing my factory recommended 190 thermostat with a 160 or 178 thermostat in hopes of having a cooler running engine?

190 is not too hot for your vehicle to be running. I'd resist going lower until you discovered what has changed in order to make the car run hotter than is has in the past. Being as old as it is, it may be time for the radiator to be boiled out, or the water pump to be replaced. Changing the thermostat to open at a cooler temperature may not stop the car from running hot anyway. If the current thermostat is opening properly, you should be right in the middle of it's (the car's) operating range. I'd look elsewhere before messing with factory specs on a good old car like that. Try back flushing the engine.